Problem: Solve for $x$ and $y$ using substitution. ${x+2y = -4}$ ${y = -x-5}$
Answer: Since $y$ has already been solved for, substitute $-x-5$ for $y$ in the first equation. ${x + 2}{(-x-5)}{= -4}$ Simplify and solve for $x$ $x-2x - 10 = -4$ $-x-10 = -4$ $-x-10{+10} = -4{+10}$ $-x = 6$ $\dfrac{-x}{{-1}} = \dfrac{6}{{-1}}$ ${x = -6}$ Now that you know ${x = -6}$ , plug it back into $\thinspace {y = -x-5}\thinspace$ to find $y$ ${y = -}{(-6)}{ - 5}$ $y = 6 - 5$ $y = 1$ You can also plug ${x = -6}$ into $\thinspace {x+2y = -4}\thinspace$ and get the same answer for $y$ : ${(-6)}{ + 2y = -4}$ ${y = 1}$